This invention relates to the art of electrostatographic copying, an electrostatographic photosensitive device and more particularly to a method of fabricating such a device comprising particulate trigonal selenium in a polymeric matrix.
The art of electrostatographic copying, originally disclosed by C. F. Carlson in U.S. Pat. No. 2,297,691, involves as an initial step, the uniform charging of a plate comprised of a conductive substrate normally bearing on its surface a non-conductive barrier layer which is covered by a layer of photoconductive insulating material. This is followed by exposing the plate to activating radiation in imagewise configuration which results in dissipation of the electrostatic charge in the exposed areas while the non-exposed areas retain the charge in a pattern known as the latent image. The latent image is developed by contacting it with an electroscopic marking material commonly referred to as toner. This material is electrostatically attracted to the latent image which is, by definition, in the configuration of those portions of the photoreceptor which were not exposed to the activating radiation. The toner image may be subsequently transferred to paper and fused to it to form a permanent copy. Following this, the latent image is erased by discharging the plate and excess toner is cleaned from it to prepare the plate for the next cycle. Typically, the photosensitive device is in the form of a cylindrical drum generally referred to as the photoreceptor.
A photoconductive material which has had wide use as a reusable photoconductor in commercial xerography comprises vitreous or amorphous selenium. Vitreous selenium in essence comprises super cooled selenium liquid and may readily be formed by vacuum evaporation by cooling the liquid or vapor so suddenly that crystals of selenium do not have time to form. Although vitreous selenium has had wide acceptance for commercial use in zerography, its spectral response is limited largely to the blue-green portion of the electromagnetic spectrum which is below about 5200 Angstrom units. In addition, the preparation of vitreous selenium by vacuum deposition requires a significant capital expenditure for vacuum coating apparatus and closely controlled process parameters are required in order to obtain a photoconductive layer having the desired electrical characteristics. In general, one requirement of a photoconductor such as vitreous selenium is that its resistivity should drop at least several orders of magnitude in the presence of activating radiation in comparison to its resistivity in the dark. Also, the photoconductive layer should be able to support a significant electrical potential in the absence of radiation.
Selenium also exists in a crystalline form known as trigonal or hexagonal selenium which is well known to the semi-conductor art for use in the manufacture of selenium rectifiers. In the crystalline trigonal form, the structure of the selenium consists of helical chains of selenium atoms which are parallel to each other along the crystalline c-axis. Trigonal selenium is not normally used in xerography as a homogeneous photoconductive layer because of its relatively high electrical conductivity in the dark, although in some instances trigonal selenium can be used in binder structures where trigonal selenium particles are dispersed in a matrix of another material such as an electrically insulating resin, an electrically active organic material, or a photoconductor such as vitreous selenium. U.S. Pat. Nos. 2,739,079 and 3,692,521 both described photosensitive members utilizing small amounts of crystalline, trigonal selenium contained in predominantly vitreous selenium matrices. In addition, copending U.S. patent application Ser. No. 669,915 describes a special form of red-hexagonal selenium suitable for use in binder structures in which finely divided trigonal selenium particles are contained in a resin binder matrix. These types of devices are normally prepared by mixing finely divided trigonal selenium in a solution of the polymer and coating the resulting dispersion onto a substrate. Removal of the solvent provides the finished layer.
It would be desirable, and it is an object of the present invention, to provide a process for the preparation of a layer of particulate trigonal selenium dispersed in a polymeric matrix wherein the selenium is generated in situ.
A further object is to provide a novel method for the preparation of an electrostatographic photoreceptor which comprises a layer of particulate trigonal selenium dispersed in a polymeric matrix.